Compressible and expansible chambers

ABSTRACT

The disclosure relates to compressible metallic bellows of the type which comprise a plurality of axially aligned, concave sheet metal annuli joined to one another at their inner and outer margins respectively so that the concavities of the annuli are alternately facing towards and away from one another, in which the radial cross-sectional shape of each annulus is such as to afford high volumetric compression ratios, impactless closing of such bellows and freedom from resonance effects when in dynamic use.

United States Patent McNamee COMPRESSIBLE AND EXPANSIBLE CHAMBERS Aug.5, 1975 3,224,344 12/1965 Baumann et a1 .1 267/162 PrimaryE.rumilierJames B. Marbert Attorney, Agent, or Firm-Allison C. Collard[57] ABSTRACT The disclosure relates to compressible metallic bellows ofthe type which comprise a plurality of axially aligned, concave sheetmetal annuli joined to one an other at their inner and outer marginsrespectively so that the concavities of the annuli are alternatelyfacing towards and away from one another, in which the radialcrosssectional shape of each annulus is such as to afford highvolumetric compression ratios, impactless closing of such bellows andfreedom from resonance effects when in dynamic use.

5 Claims, 15 Drawing Figures PATENTEB AUG 1 75 SHEET PATENTED 55375 3'897' 942 SHEET 4 COMPRESSIBLE AND EXPANSIBLE CHAMBERS This inventionrelates to compressible metallic bellows, to methods of manufacturingsuch bellows and to machines such as, for example, compressors, engines,actuators and the like, in which such bellows replace and perform theduties of a cylinder and piston. in particular the invention relates tometallic bellows of the type which comprise a number of axially aligned,concave, sheet metal annuli joined one to another at their outer andinner margins respectively so that the concavities of the annuli arealternately facing towards and away from one another.

Such bellows are primarily intended for use in compressors, although arenot restricted to such, and must therefore satisfy a number ofrequirements which are:

l. Able to withstand long-term continuous mechanical flexure.

2. Capable of withstanding high internal (or external) pressures.

3. Permit high volumetric efficiencies and high compression ratios.

4. Be free from natural resonance effects.

5. Be free from impact upon closure, i.e., free from high velocitycontact between adjacent portions of the bellows during any part of thebellows stroke.

Compressors which employ metallic bellows have been described in BritishPatent Specification No. 438,404, in which the bellows are compressibleand which comprise a series of concave annular discs of the typedescribed above, the shape of which is not defined but which areillustrated as conically dished discs. There is no teaching in thespecification as to how the stringent requirements of a compressor maybe met by the bellows, or as to how the shape of the disc concav ity maybe varied in order to modify a bellows performance towards meeting thestated requirements.

Bellows which possess some of the desired characteristics are describedin British Patent Specification No. 474,753 and which comprise a seriesof thin, flat, annular metal discs alternately welded together at theirin nermost and outermost margins. Such bellows are nor mally closed(i.e., the surfaces of adjacent discs are fully contiguous) and in useare extended, rather than compressed as are bellows of the invention.However, the volumetric efficiency of such bellows would appear to behigh when internal pressures are low but, as FIG. 8 of the specificationshows, it would appear difficult to obtain contiguity of adjacentinternal disc surfaces at high pressures, furthermore, considerablestress would also appear to be generated in the outermost margins of thediscs, leading to metal fatigue and eventual rupture of the bellows.

British Patent Specification No. 759,849 describes compressor bellowssimilar to those of British Patent Specification No. 474,753 except thatthese are compressed by fluid pressure acting upon and externallythereto, thus reducing the pressure difference across the bellows tonegligible proportions. Accordingly, the bellows described can be madefrom thin, freely bendable material such as for example light gaugecopper.

Compressible bellows comprising a series of conically dished annulardiscs have been proposed, such as in previously mentioned British PatentSpecification No. 438,404, which bellows suffer from several practicaldisadvantages, namely:

I. When a conical annular disc, i.e. like a Belleville washer, isaxially compressed until the inner and outer perimeters lie in the sameplane, the disc itself does not assume a planar condition, as may beimagined, but in radial cross-section adopts a wavelike curve. Bellowscomprising such conical discs can only be compressed sufficiently forcomplete radial contiguity of the internal surfaces, by the applicationof a large axially directed pressure acting over the whole externalsurface of the two ends of the bellows. Furthermore, the stresses raisedin each disc when repeatedly so flattened would be sufficient to ensurerapid failure due to fatigue.

2. Such bellows have a substantially constant spring rate andaccordingly have a natural resonance which can limit the operationspeeds of a compressor using the bellows.

3. As the bellows are compressed, flexural motion is abruptly arrestedas maximum compression is achieved, i.e., as the adjacent discs cometogether, albeit in distorted form.

It is such disadvantages that the invention seeks to overcome by theexpedient of providing bellows in which the radial cross-sectional shapeof each concave sheet metal annulus in the unstressed condition is suchthat when the inner and outer perimetric margins are moved axiallyrelative to one another so that they lie in the same plane, the adjacentsurfaces of any two adjacent annuli are contiguous between the twomargins and substantially flat.

In practice, where adjacent annuli are interattached at their margins,the joint is made over a radial margin length substantially greater thanthe combined thickness of the two annuli. In order to facilitatejoining, the inner and outer margins are flat, lying in planes parallelwith each other, and may be joined one to another by brazing, soldering,welding and the like.

In such an arrangement, when a bellows is compressed, the joinedmargins, although moving axially, remain flat and parallel with oneanother.

It has been found that, if each annulus of such a bellows is pre'formedto a shape identical with that shape which would be generated in asimilar but flat annulus when the two margins thereof are axially andundeformably displaced whilst remaining parallel to one another, thebellows which result exhibit advantageous characteristics.

For example, as an annulus so shaped is axially compressed, the curve ofits shape becomes progressively shallower until, when the two marginsare in the same plane, the annulus is flat. Accordingly, a bellowsfabricated from such annuli may be fully compressed, all fluid beingexpelled from between the annuli, thus affording high compressionratios. Furthermore, increasing internal pressure modifies the action ofthe bellows causing rolling to occur between the external surfaces ofadjacent annuli, commencing at the inner margin and moving radiallyoutwards until, at full compression, complete contiguity of the surfaceresults. An immediate effect of this rolling action is to cause thestiffness of the bellows to increase as the axial length reduces, thusobviating natural resonance effects. However, when no pressuredifference exists across the bellows, the stiffness is substantiallyconstant.

Also, it can be shown that the circumferential and radial membranestresses, and circumferential bending stresses of each fully compressedannulus are negligible, unlike in other known bellow configurations, thedominant stress being the radial bending stress which is greatestadjacent the inner margin and is conveniently of such sign as to opposeand tend to cancel internal pressure-induced stresses. The bellows cantherefore withstand higher internal pressures than comparable knownbellows.

Although bellows fabricated from annuli shaped in accordance with anaturally deformed flat annulus as described, which shape is hereinafterreferred to as the natural shape," have improved characteristics and areembraced by the invention, t has been found that simple modifications tothe natural shape afford further improvements and enable a particularbellows characteristic to be predetermined. For example, increasing thecurvature in the region of the outer margin of each annulus affords (a)a greater bellows stroke, (ba radial rolling action from inside tooutside of adjacent annuli, even in the zero pressure differencecondition, with resulting change of bellow stiffness with axial length,and (c) an increase in radial bending stress towards the outer margin ofeach annulus when compressed (with out appreciably increasing themaximum stress of the inner margin) thus permitting higher internalpressures which tend to be cancelled by such bending stress. Themodification to shape is, of course, not so great that the outermostradial bending stresses exceed the permissible fatigue stress of thebellows material.

The effect of decreasing the curvature of the annular region adjacentthe outer margin reduces the radial bending stress adjacent the outermargin of each annulus and thus permits bellows so modified to withstandhigher external pressures and, furthermore, results in a rolling actionbetween adjacent internal annuli surfaces commencing at the outer marginand moving radially inwards, even in the zero pressure differencecondition, until complete contiguity of the surfaces occurs. Again, thestiffness of bellows so modified varies with axial length but, thestroke of the bellows is reduced by an amount related to the decrease incurvature.

Accordingly the invention provides compressible metallic bellows of thetype specified in which each annulus is, in its neutral condition, inradial cross-sectional shape curved between a flat outer margin whichlies in one plane and a flat inner margin lying in a plane parallel tothat of the other margin, the curve being such as to resemble thatnatural curve, which would be generated if the inner and outer marginsof a dimensionally similar flat annulus of the same metallic materialwere axially and undeformably displaced whilst remaining parallel withone another.

In one aspect of the invention the radial crosssectional shape of eachannulus corresponds with that of a flat annulus deformed as described.This natural shape in cross-section is ogival, sinusoidal or like anelongated S, having a positive curvature between the outer margin and apoint somewhere between the two margins at which point the curvature hasreduced to zero, and continuing with negative curvature from that pointtowards the inner margin, each margin being tangential to its respectivecurve. The choice of terms positive" and negative" curvature being quitearbitrary and chosen to assist in the definition of the modifications tothe natural annular shape yet to be described.

Further according to the invention each annulus of the bellows may beshaped so as to correspond with the natural shape with the exception ofthat annular region of positive curvature, the curvature of which regionis increased throughout its radial length.

Also in accordance with the invention each annulus may be shaped so asto correspond with the natural shape with the exception of thepositively curved region, the curvature of which region is decreasedthroughout its radial length.

The invention also provides for a method of manufacturing compressiblemetallic bellows of the type specified and which comprises the followingsteps:

1. Forming the annuli to the desired shape with concurrent or subsequentheat treatment thereof.

2. Joining together the inner and outer margins of adjacent annuli byelectron-beam welding, the beam being directed so as to impinge radiallyupon the abutting edges of the respective margins whilst rotating theannuli relative to the beam, the beam being of such dimensions andintensity as to produce a seam of radial depth preferably greater thanthe combined thickness of the abutting margins.

Bellows according to the invention may be adapted for actuation byexternal fluid pressure by the provision of annular spacers interposedbetween and joined with the inner margins of adjacent pairs of annulicojoined at their outer margins. The use of such spacers provides accessfor pressurised external fluid to the outer surfaces of the annuli atall stages of bellows compression, thus affording faster closing ofbellows so adapted. The provision of a longer junk piston within thebore of such a bellows is desirable, to provide the desired overallcompression ratio, due to the increase in length and resulting increasein free volume of the bellows when fully compressed, following the useof spacer rings.

A similar arrangement of spacer rings joining with the outer margins ofadjacent annuli pairs cojoined at their inner margins is also embracedby the invention.

Further according to the invention, compressors are provided whichinclude a bellows as described herein.

Examples of the invention will now be described with reference to theaccompanying drawings which are as follows:

FIG. 1 shows in cross-section a bellows of the kind specified; 1

FIG. 2 illustrates in part-section a portion ofsuch bellows constructedfrom naturally shaped annuli;

FIGS. 3 and 4 show what is meant by the term naturally shaped" annulusand how such an annulus may be formed;

FIG. 5 illustrates in part the bellows of FIG. 2 when partiallycompressed,

FIG. 6 shows the same bellows fully compressed;

FIG. 7 depicts a naturally shaped annulus and also shows one in dottedoutline which has increased positive curvature;

FIG. 8 again shows the naturally shaped annulus of FIG. 7, but thedotted outline in this instance depicts an annulus having decreasedpositive curvature;

FIG. 9 shows a partially compressed bellows in which the annuli haveincreased positive curvature;

FIG. 10 is of a similarly compressed bellows having annuli of decreasedpositive curvature;

FIGS. 11 and I2 illustrate bellows in which cojoined pairs of annuli arespaced apart by rings;

FIGS. 13 and I4 illustrate diagrammatically a compressor in which thebellows of FIG. II are used to compress a fluid; and

FIG. shows another compressor driven by a cam shaft and which employsbellows having increased positive curvature.

Referring to FIGS. 1 and 2, a bellows of the kind specified comprises aseries of concave sheet metal annuli 1 arranged in axial series as shownand welded together at their respective inner and outer margins, 4 and 5respectively, these inner and outer margins being flat and lying inparallel planes. The radial length of each weld 4A and 5A beingsubstantially that of the width of its associated margin 4 and 5. Suchdepth of weld may be obtained by use of electron beam welding.

In order to define the shape of each annulus l, we now refer to FIGS. 3and 4. FIG. 3 shows a flat annular disc of sheet metal 6 clamped at itsouter margin be tween the two halves of a rigid ring 7 and clamped atits inner margin between the two halves of a rigid cylindrical clamp 8.When clamp 8 is deflected axially relative to ring 7 by an amount d,whilst maintaining parallelity between the inner and outer margins ofdisc 6 without exceeding the elastic and fatigue limits of the material,the disc in radial cross-section assumes a shape defined hereinbefore asthe natural shape." If the deflection d is now maintained and thusdeformed disc 6 together with clamps 7 and 8 heated to the region of thestress-relieving temperature of the disc material, upon subsequentcooling, the disc 6 will have permanently assumed the natural shape.

In designing bellows which use such permanently formed discs 6,deflection d is made of such magnitude that, when a disc is fullycompressed, i.e., until it is flat, the permissible fatigue stress ofthe disc material is preferably not exceeded. The maximum stress thuscreated, a radial bending stress, resides adjacent the inner margin ofthe disc.

The bellows illustrated in part in FIGS. 2, 5 and 6 comprises discs orannuli 1 formed with natural shape, which shape permits such bellows tobe fully compressed so that the facing internal surfaces 2 and facingexternal surfaces 3 of the annuli are flat and contiguous. FIG. 5illustrates such bellows partially compressed and, as can be seen, theannuli l are approaching a flat condition without buckling or otherwisebecoming deformed as would occur had the annuli not possessed thenatural shape. FIG. 6 shows the same bellows in fully compressedcondition.

It should be mentioned that the shape of each annulus l as shown in FIG.5 is that which would be assumed thereby in the absence of internalpressure within the bellows, and that in such absence the bellows formedfrom naturally shaped annuli possess a constant spring rate orstiffness.

The effect of internal pressure within the bellows as illustrated inFIGS. 2, 5 and 6, as would be the case if such bellows were employed tocompress a fluid, would be to squeeze together adjacent annuli so thatthe facing external surfaces 3 thereof become progressively contiguous,a rolling action occurring between the surfaces 3 commencing adjacentmargin 4 and moving towards margin 5 (FIG. 9 illustrates a similareffect but which is due to the modified shape). Upon full compression,contiguity is once more achieved as shown in FIG. 6.

The effect of the internal pressure is to generally increase the stresswithin the annuli, but as compression of the bellows increases, theensuing outward rolling action provides natural support between annuliso that stresses, where contiguity of surfaces 3 has occurred, do notexceed the normal radial bending stresses. However, the pressure stressunder such circumstances can greatly exceed the radial bending stressadjacent the outer margin 5 and limit the usefulness of the bellows.

Since this outer radial bending stress is of opposite sign to that ofinternal pressure stress, by deliberately increasing the bending stressadjacent outer margin 5 of each annulus I when the bellows are partiallyor wholly compressed, it becomes possible to tolerate higher internalpressures. In order to achieve such increase in outer radial bendingstress the curvature of each annulus is modified as will now bedescribed with reference to FIG. 7.

The cross-sectional radial shape of a naturally shaped annulus I isillustrated in solid outline in FIG. 7, and which is composed of flatouter margin 5 tangential to the curve which commences at A, movingtowards inner margin 4, and continuing with decreasing positivecurvature until a point C is reached where the curvature has reduced tozero, at which point the curvature changes sign and now continuestowards the inner margin 4 with increasing negative curvature untilpoint B is reached, at the boundary of margin 4, where the inner marginis tangential to the curve. Point C is not necessarily mid-way between Aand B, but displaced somewhat towards B since the curve is that of anannular disc rather than that of a linear beam.

A line drawn through C, normal to the annulus surface at that pointwould cut two parallel lines which respectively pass through points Aand b, at some angle When the positive curvature, i.e., that betweenpoints A and C, is generally increased without substantially changingthat curvature lying between C and B, the shape of the annulus assumesthat illustrated by dotted line A C and the solid line C B. Point C' ismove somewhat nearer B, and the normal line passing through C now cutsthe two mentioned parallel lines with increased angle 0'.

A bellows made from annuli having such a modified shape would, duringcompression, possess a rolling action from inside to outside betweensurfaces 3, and FIG. 9 illustrates such a bellows when partiallycompressed, with or without internal pressure, although the presence ofinternal pressure exagerates the action.

A greater radial bending stress is produced at point A (FIG. 7) (this isnot strictly a point but a circumferential line) when such bellows arecompressed, the modification to shape being such that the maximumpermissible fatigue stress is not exceeded. Conversely, if the region ofpositive curvature in each annulus l is generally decreased, asillustrated by the dotted line A" C" in FIG. 8, angle 0 now beingdecreased to 9", a rolling action from outside to inside between facinginterval surfaces 2 of adjacent annuli I occurs (see FIG. 10) thussqueezing internal fluid radially inwards, and the radial bending stressat point A is decreased. Since stresses induced in such bellows byexternal pressure are of the opposite sign at the inner margin as thebending stresses, increased bending stress will permit higher externalpressures to be achieved. Annuli of decreased positive curvature aretherefore employed in bellows which are compressed by the action ofexternal pressure as in the example of compressor to be described inrelation to FIGS. 13 and I4.

The rolling action induced by this modification to shape results inimproved inward scavenging of gases and (b) mutual support betweencontiguous surfaces so that pressure induced stresses are not domi nant.

Where bellows are to be compressed by the action of external pressure.as the bellows approaches the fully closed condition, the compressingaction becomes less effective and therefore slower as access of thepressurising fluid to external surfaces 3 of annuli l is restricted. Amodification to such bellows is shown in FIG. 11, in which cojoinedpairs of annuli are spaced apart by rings II) which are joined to therespective inner margins 4 of adjacent cojoined pairs. The closingaction of the individual pairs of annuli remain unchanged, but theoverall length of bellows is increased.

Similarly, where a compressed bellows is to be extended by the use ofinternal pressure, such as in an actuator for example, spacer rings suchas rings II of FIG. 12, may be used to separate pairs of annuli I, eachcojoined at their inner margins 4, the rings being inserted between theouter margins 5 of adjacent pairs.

A compressor, such as for compressing air will now be described withreference to FIGS. 13 and 14, and which comprises a chamber inside whicha bellows 26 is joined to one end thereof, and which chamber is providedwith a cylindrical portion 21 in which slides a piston 22. The free endof bellows 26 is closed by means of a circular plate 28 to which it iswelded and which plate carries a junk piston 29 on its surface withinand concentric with the bellows 26. The bellows 26 is similar to thatillustrated in FIG. II, and has spacer rings 27 corresponding to ringsI0 of FIG. 11. The axial length ofjunk piston 29 is approximately thatof the three rings 27 combined plus the total thickness of annuli, andof slightly smaller overall diameter than the internal diameter of therings. Air inlet and outlet valves, diagrammatically illustrated bydiscs 30 and 31 co-operating with respective ports 32 and 33 in thechamber 20 wall, are provided for admission of air into, and expulsionof air from, the internal volume of bel lows 26.

The space defined by the piston 22, cylinder 2I, chamber 20, bellows 26and endplate 28 is filled with a fluid 25 such as oil so that, as piston22 is caused to reciprocate within cylinder 21 by means of crank 23 andconnecting rod 24, the bellows 26 are alternately compressed andexpanded by the action of fluid 25 thereupon.

FIG. I shows the bellows 26 in the fully compressed condition, andillustrates how the provision ofjunk piston 29 serves to reduce the freevolume of the compressed bellows, and thus affords, together with thecomplete closure of each pair of bellow annuli, high compression ratios.

Bellows contemplated by the invention are made from quite thick sheetmaterial compared with bellows of similar dimensions intended forsimilar use, for example an external annulus diameter of 8 inches andinternal diameter of 4 inches, the thickness of each annulus is between0.032 and 0.036 inches for martensitic stainless steel, permittinginternal pressures of ISO to 500 lbs/ins to be obtained in compressionwithout exceeding the fatigue stress limit, and permitting greater than[0 flexural cycles to be obtained. For bellows compressed by externalfluid pressure, it is believed that internal pressures of the order of3,000 atmospheres may be tolerated whilst nondynamic bellows areexpected to tolerate even higher pressures. It is anticipated thatsomewhat lower pressures may increase still further the fatigue. Suchstiffness can be utilized to provide the restoring force which extendsthe bellows after compression, and FIG. 15 illustrates diagrammaticallya compressor in which a face-cam is used to compress the bellows in onedirection, the bellows being free to extend as a result of theirstiffness when so per mitted by the cam. A face-cam 45 is driven torotate by a shaft 47, any axial thrust on the cam 45 being carried bythrust-bearing 46 and casing 48. A roller 43, attached eccentrically toa pillar 42 which is aligned with the axis of shaft 47, and free toslide axially in guide 44, bears upon cam 45 and follows the contourthereof as the cam rotates. The roller 43 accordingly imparts areciprocating motion to pillar 42 which in turn alternately compresses abellows 40, to which it is attached via bellows end-plate 41, andpermits it to extend. the spring force of the bellows 40 maintaining theroller 43 thrust against the face-cam 45.

Where reference is made in this specification to increasing ordecreasing the positive curvature of an annulus radial cross-section, itis understood that this modification is relative to that section havingnegative curvature and that similar results may be obtained by changingthe degree of negative curvature relative to that which is positive.

Furthermore, although reference is made throughout the specification tocircular annuli, the invention embraces bellows having elements ofdifferent shape such as for example oval. Also, annuli comprising two ormore thicknesses of sheet metal, i.e., for bellows known as multiplybellows, are included in the scope of the invention.

In conclusion the readers attention is directed to U.S. Pat. No.3,090,043 wherein bellows, such as are known to have been used incryogenic gas expansion engines, are described having such shape as topermit complete bellows closure, but which rely upon thin, multiplydiscs together with a tortuous cross-sectional shape to avoiddestructive stress and undue buckling of the annular elements when fullycompressed. US. Pat. No. 3,090,403 clearly indicates some of theproblems which the invention seeks to conquer.

I claim:

1. Compressible metallic bellows comprising a plurality of concave,sheet metal annuli axially aligned and joined one to another at theirinner and outer margins respectively so that the concavities of theannuli are alternately facing towards and away from one another, inwhich each annulus in its neutral condition comprises:

at least one flat outer margin lying in a first plane;

at least one flat inner margin lying in another plane parallel with saidfirst plane;

a curved portion disposed between and continuous with said flat innerand outer margins;

the shape of the curved portion being such that over its length betweensaid inner and outer margins, a sinusoidal curve is generated so thatwhen the bellows is axially displaced, the shape of the curved portionconforms to a sinusoidal curve over at least a part of its length, andwhen said bellows are compressed, the curved portions are flattened withrespect to each other.

10 said margin.

5. Bellows according to claim 4 wherein the election beam is directed soas to impinge radially upon the abutting edges of the respective marginswhile the annuli are rotated relative to the beam, the beam having anintensity and dimension so as to produce a seam of radial depth greaterthan the combined thickness of the abutting margins.

1. Compressible metallic bellows comprising a plurality of concave,sheet metal annuli axially aligned and joined one to another at theirinner and outer margins respectively so that the concavities of theannuli are alternately facing towards and away from one another, inwhich each annulus in its neutral condition comprises: at least one flatouter margin lying in a first plane; at least one flat inner marginlying in another plane parallel with said first plane; a curved portiondisposed betweeN and continuous with said flat inner and outer margins;the shape of the curved portion being such that over its length betweensaid inner and outer margins, a sinusoidal curve is generated so thatwhen the bellows is axially displaced, the shape of the curved portionconforms to a sinusoidal curve over at least a part of its length, andwhen said bellows are compressed, the curved portions are flattened withrespect to each other.
 2. Bellows according to claim 1 in which adjacentinner margins are axially separated by and joined with an annularspacer.
 3. Bellows according to claim 1 in which adjacent outer marginsare axially separated by and joined with an annular spacer.
 4. Bellowsaccording to claim 1 in which said inner and outer margins are joinedtogether by an electron beam weld, the depth of each weld in radialdirection being substantially the radial length of the associated saidmargin.
 5. Bellows according to claim 4 wherein the election beam isdirected so as to impinge radially upon the abutting edges of therespective margins while the annuli are rotated relative to the beam,the beam having an intensity and dimension so as to produce a seam ofradial depth greater than the combined thickness of the abuttingmargins.